1. Field of the Invention
This invention in general relates to optical systems and, in particular, to a photographic objective system which utilizes a transversely movable member of preferred shape for maintaining the focal setting of the objective over a large range of object distances.
2. Description of the Prior Art
From the earliest use of hand cameras, it has been recognized that a change in object distance with respect to the location of the camera objective lens causes an inevitable and easily calculated change in image distance which, if not compensated in some way, leads directly to a degradation of image quality over the chosen field of view. Everyone who has made use of photographic objectives becomes aware of this fundamental fact, and indeed camera manufacturers have adopted several convenient means for bringing the aerial image and the sensitive photographic film or coated glass plate into registration.
The most natural means, one employed from the beginning, is simply the technique of moving the position of the photographic objective along the optical axis for the purpose of focusing. Generally, the film plane remains stationary. However, there are cameras and certainly telescopes with movable film or plate holders, particularly where the photographic equipment is large and cumbersome. In either case, the distance from objective to film is changed in such a way that the image can be focused onto a ground glass and thereafter onto a substituted photographic emulsion.
In some cameras, particularly in the modern era, it has proved to be convenient to restrict the focusing movement to but a portion of the optical system, generally but a single element or component. The movable element or component, however, is a mixed blessing inasmuch as the image quality may suffer as a result of the displacement of the element or component from its optimum position. Various aberrations that have been minimized or balanced for good image quality under average conditions reappear or become larger on displacement of an element or component. Both lateral and longitudinal chromatic aberrations may reappear, along with an enhancement of spherical aberration, coma, and astigmatism. However, careful design has often resulted in practicable systems with substantial range in object distances and even in magnification, as for example, the various forms of zoom systems now generally available.
Other forms of focusing have also been introduced. It is possible to interchange lens elements providing a discrete change in dioptric powers to provide a reduced focusing range for each, within which individual range the image quality can remain reasonably stable. The sequence of focusing ranges can then be made to overlap in such a way that the convertible system can be used over a large range of object distances. This technical means becomes all the easier if the focusing interchangeable elements are inherently of low dioptric power, whether positive or negative. In this way, the weak element in use at any given time interferes only slightly with the image quality and indeed may be used to improve the quality if suitably located and shaped. With moldable elements use can be made of an aspheric "touch-up" to improve the image quality selectively within the individual range. If the dioptric lens elements are mounted onto a rotor or disc for easy interchange, the rotor can be referred to as a set of Waterhouse elements. Waterhouse discs have also been used from long ago for aperture control and for insertion of readily interchangeable filters.
Still another form of focusing involves the use of liquid filled flexible cells which with changing pressure can be made to perform weak dioptric tasks such as focusing. Ordinarily, the sagittae associated with dioptric focusing of hand camera objectives are very small, whether positive or negative, and for the usual focal lengths can be measured in but a few dozens of microns. It is necessary, however, that the deformed flexible cell provide sufficiently smooth optical surfaces for acceptable image quality after focusing has been performed.
Still another form of focusing has been introduced in U.S. Pat. No. 3,305,294 issued to L. W. Alvarez on Feb. 21, 1967. In this device a pair of deformed plates are moved transversely in equal but opposite displacements. The plates have the same shapes but are opposed such that in the "null" position the variation in thickness cancels and the two plates used together have zero dioptric power. The polynomial expressions needed to define the common aspheric shape is strongly dependent on cubic terms in a power series in two variables. The polynomial coefficients are carefully chosen to allow the plates to simulate by transmission and refractions the performance of a weak dioptric lens. When the plates are moved transversely with respect to one another, the net effect is used to simulate a weak bi-convex or bi-concave simple element thereby providing for a continuous range of low dioptric powers.
It is a primary object of the present invention to provide a photographic objective system having but one transverse movable member for maintaining the focal setting of an objective system over a large range of object distances.
It is another object of the present invention to provide a photographic objective of the above type for use at relatively fast aperture ratios.
Other objects of the invention will in part be obvious and will in part appear hereinafter. Accordingly, the invention comprises the optical system possessing the construction, combination and arrangement of elements which are exemplified in the following detailed description.